Global Properities of the Interstellar Medium in Nearby Galaxies

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Far-infrared spectra (45-189 μm) of 36 nearby galaxies taken by the Long WavelengthSpectrometer (LWS) aboard the Infrared Space Observatory (ISO) were analyzed to investigatethe general properties of interstellar matter in galaxies. Far-infrared forbidden lines, such as[CII]158 μm, [OI]63 μm, and [NII]122 μm, were detected in most of the sample galaxies. The lineintensities of [CII]158 μm and [NII]122 μm relative to the far-infrared intensity (FIR) decrease asthe far-infrared color becomes bluer, while the ratio of the [OI] intensity to FIR does not showa systematic trend with the color. We estimate the contribution from photodissociation regions(PDRs) to the observed [CII]158 μm intensity, assuming that all the [OI]63 μm and far-infraredcontinuum emission comes from the PDRs. The results indicate that the fraction of [CII]158 μmemission arising from the PDRs ranges from 30% to 80% among the sample galaxies. The rest of theemission is suggested to originate from the extended low-density warm ionized medium (ELDWIM).The derived neutral hydrogen gas density increases as the far-infrared color becomes bluer, beingcompatible with the idea that active galaxies have dense gas in their interstellar medium. Thecontribution from ELDWIM to [CII]158 μm/FIR does not show any systematic trend with thefar-infrared color. The present results suggest that the relation between [CII]158 μm/FIR and thefar-infrared color can be attributed to the decrease in [CII] emission from the PDRs owing to theincrease in collisional de-excitation rather than the decrease in the photoelectric heating efficiency.
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Global Properities of the Interstellar Medium in Nearby Galaxies

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